The present invention relates to a process for selectively etching one or more oxide layers on a surface of a substrate, and more particularly, to such a process in semiconductor device fabrication to etch silicon oxides.
In the manufacture of semiconductor devices, oxides of silicon are used in many different forms and for different applications. Dense, thermally grown or chemically deposited oxides may find use as dielectric films and insulating layers. Typical of such oxides is the class of tetraethylorthosilicate (TEOS) derived oxides.
Other less dense forms of silicon oxides are also used in semiconductor device fabrication where planarized insulating layers are desired. Examples of these types of oxides include doped oxides such as phosphosilicate glass (PSG), borosilicate glass (BSG), borophosphosilicate glass (BPSG), and boron or phosphorous-doped TEOS. Spin-on glass (SOG) is another porous oxide which is used, especially where planarization is desired.
Many semiconductor manufacturing processes require selective etching to remove one form of silicon oxide (typically a more porous form such as BPSG) in preference to another silicon oxide (typically a dense form such as TEOS) or other material (such as silicon). Where there is a desire for selective etching of different forms of silicon oxides, typically hydrogen fluoride (HF) is used as the primary etchant. However, wet etching using aqueous solutions of HF is not very selective, etching both dense and more porous forms of silicon oxides at similar rates. The art has moved to the use of vapor phase HF etching processes to achieve greater selectivity.
For example, Bergman, U.S. Pat. Nos. 5,235,995, 5,238,500, and 5,332,445 teaches a vapor etch process using a homogeneous mix of HF and water vapor as the etchant gas. Grant et al, U.S. Pat. Nos. 5,234,540 and 5,439,553, teach a vapor phase etching process using HF and an alcohol or organic acid. Mehta, U.S. Pat. No. 5,635,102, teaches a selective etching process which exposes the silicon oxides to alternating pulses of HF gas and inert gas causing selective etching of a porous silicon oxide layer (BPSG) in preference to a dense silicon oxide layer (TEOS). However, while such processes may be selective, some have resulted in an undesirable non-uniform etching of the oxide layers.
One example of a semiconductor fabrication process which requires selective etching of different silicon oxides is the formation of stacked capacitor structures to be used in storage devices such as high-density dynamic random access memories (DRAMs). Such structures are formed using a large silicon wafer as a substrate. Fabrication of these devices requires not only a highly selective etching process, but also one which uniformly etches the oxide layers across the surface of the wafer.
Accordingly, there remains a need in this art for an etching process for oxides having differing densities which is not only highly selective, but also produces uniform etches.
The present invention meets that need by providing a process for etching oxides having differing densities which is not only highly selective, but which also produces uniform etches. In accordance with one aspect of the present invention, a process is provided and includes the steps of providing an oxide layer on a surface of a substrate, exposing the oxide layer to a liquid comprising a halide-containing species, and exposing the oxide layer to a gas phase comprising a halide-containing species. Preferably, halide-containing species is selected from the group consisting of HF, NF3, CIF3, and F2. In a preferred embodiment of the invention, the halide-containing species comprises HF, and the gas phase includes an alcohol.
The process of the present invention desirably is used to selectively etch a substrate surface in which the surface of the substrate includes on a first portion thereof a first silicon oxide and on a second portion thereof a second silicon oxide, with the first silicon oxide being relatively more dense than the second silicon oxide. In this manner, the steps of exposing the oxide to a liquid comprising a halide-containing species and exposing the oxide to a gas phase comprising a halide-containing species causes the second silicon oxide to selectively etch at a rate greater than the etch rate of the first silicon oxide. In a preferred embodiment, the first silicon oxide comprises a tetraethylorthosilicate derived oxide and the second silicon oxide comprises borophosphosilicate glass. The process of the invention is particularly useful where the second silicon oxide overlies the first silicon oxide, and the first silicon oxide acts as an etch stop layer.
In accordance with another aspect of the invention, a process for etching a layer of a silicon oxide on a substrate is provided and comprises the steps of providing a silicon oxide layer on the surface of a substrate, exposing the silicon oxide layer to a liquid comprising an aqueous solution of hydrofluoric acid, and exposing the silicon oxide layer to a gas phase comprising hydrofluoric acid vapor. In a preferred form, the gas phase includes an alcohol such as methanol to promote a uniform etch.
In this embodiment of the invention, the surface of the substrate preferably includes on a first portion thereof a first silicon oxide and on a second portion thereof a second silicon oxide, with the first silicon oxide being relatively more dense than the second silicon oxide. The steps of exposing the oxide to a liquid comprising an aqueous solution of hydrofluoric acid and exposing the oxide to a gas phase comprising hydrofluoric acid vapor causes the second silicon oxide to selectively etch at a rate greater than the etch rate of the first silicon oxide. Preferably, the first silicon oxide comprises a tetraethylorthosilicate derived oxide and the second silicon oxide comprises borophosphosilicate glass. Where the second silicon oxide overlies the first silicon oxide, the first silicon oxide acts as an etch stop layer.
The process of the present invention finds use in the fabrication of semiconductor devices. In one embodiment, a process for forming hemispherical grain silicon is provided and comprised the steps of forming a polysilicon or amorphous silicon layer on a substrate and exposing the layer to a gas phase comprising a halide-containing species for a time sufficient to remove any oxides thereon. Then, without exposing the layer to oxygen or an oxygen-containing gas, the layer is annealed at an elevated temperature to transform the polysilicon or amorphous silicon into hemispherical grain silicon. Preferably, the halide-containing species is selected from the group consisting of HF, NF3, CIF3, and F2. In a preferred form, the halide-containing species comprises HF, and the gas phase includes an alcohol. The annealing step is carried out at an elevated temperature of above about 200xc2x0 C.
In a preferred embodiment of the invention, the process is used to form a capacitor storage cell on a semiconductor substrate and comprises the steps of forming a first layer of a silicon oxide on the surface of the substrate and then forming a second layer of a silicon oxide on the first layer of silicon oxide, with the first silicon oxide being relatively more dense than the second silicon oxide. An opening is formed into the first and second silicon oxide layers, and a polysilicon or amorphous silicon container structure is formed having generally vertically-oriented side walls in the opening. At least a portion of the second silicon oxide layer is selectively removed by exposing the second silicon oxide layer to a liquid comprising a halide-containing species. The remainder of the second silicon oxide layer is removed by exposing the second layer to a gas phase comprising a halide-containing species, thereby exposing the side walls of the container structure. Without exposing the substrate to oxygen or an oxygen-containing gas, the container walls are annealed at an elevated temperature to transform the polysilicon or amorphous silicon into hemispherical grain silicon. The hemispherical grain silicon walls are conductively doped to form capacitor plates, and a capacitor dielectric layer is formed over the capacitor plates. Finally, a second conductive silicon layer is formed over the capacitor dielectric layer to complete the structure.
Accordingly, it is a feature of the present invention to provide a process for etching oxides having differing densities which is not only highly selective, but which also produces uniform etches. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.